1. Field of the Invention
This invention relates generally to a system and method for reducing or mitigating the effects of Doppler shift, multipath and/or noise for a mobile receiver and, more particularly, to a system and method for reducing or mitigating the effects of Doppler shift, multipath and/or noise for a television receiver on a vehicle using a map database, a GPS receiver and/or a steerable antenna.
2. Discussion of the Related Art
Conventional broadcasting of data, such as television transmission signals, typically uses analog signals. In North America, television stations will be transitioning to a digital format where the television transmission signals will typically be produced and transmitted using an advanced television standards committee (ATSC) data frame protocol, well known to those skilled in the art. With the advent of digital data broadcasting techniques, the same amount of information to be broadcast requires less bandwidth as compared to broadcasting analog signals. Therefore, broadcasting data using digital signals allows additional data to be broadcast for a given bandwidth while still delivering high quality programs. The additional data can be used for many applications, including alternative television programming and other data related services, including the transmission of weather information, traffic information and audio and video files. Further, the transition to digital television transmission enables new receiver technology.
Data is generally transmitted in the form of multiple data frames. Each data frame transmitted to a non-moving receiver typically includes a frame sync portion and a data portion. The frame sync portion allows the receiver to lock onto the received signal so that the transmitted signal can be received and deciphered by the receiver. In one technique, the frame sync portion locks onto the receiver with the received signal by setting equalization coefficients in the receiver.
After a certain period of time, a non-moving receiver goes out of synchronization with the incoming signals. This happens as a result of data dispersion caused by multipath fading during signal propagation. Multipath fading causes errors that affect the quality of reception. The errors are due to intersymbol interference (ISI). ISI is a form of distortion of a signal in which one symbol interferes with subsequent symbols. This is an unwanted phenomenon as the previous symbols have a similar effect to noise, thus making the communication less reliable. Equalizers can be used to correct ISI. Hence, in the case of non-moving receivers, the receiver is locked with the received signals using the frame sync after every predefined interval of time for continuous reception of the data without any significant loss.
For a moving receiver, the data frame suffers additional dispersion caused by the Doppler shift along with the distortion caused by ISI due to the multipath fading. Therefore, a receiver in a moving vehicle needs to resynchronize with the incoming data frames more frequently as compared to a stationary or non-moving receiver for successful reception of the data without any significant loss. However, due to the time interval between the transmission of the sync data, receivers in a moving vehicle are not able to adjust the equalizer coefficients as fast as desired, which causes loss of synchronization and loss of a significant portion of the data.
Further, reception quality is reduced by multipath effects. Multipath effects occur as a result of the receiver receiving the broadcast signal from the broadcast tower for multiple directions as a result of the broadcast signal being reflected off of objects between the tower and the receiver, such as buildings, terrain, etc. Because the several signals are received by the receiver at different times, they are out of phase with each other, which affects the ability of the receiver to process the signals.
Current and future vehicles may include map databases, traffic information systems, entertainment systems, etc. that require continuous, or near continuous, updating in order to be effective. Typically, updating techniques for these types of systems need to be low cost in order to be marketable. Data casting using ATSC digital television is one potential technique for providing such a low cost technique. Further, it may be desirable in some vehicles or vehicle systems to receive broadcast television signals using ATSC signals.